In one known structure of lighting devices using light emitting diodes (LED), laser diodes (LD), etc. as light sources, white light is obtained by passing light from, for example, a blue LED or a blue LD through a fluorescent material that emits light of a color complementary to blue. The fluorescent material used may be a resin, glass, etc. In recent years, the brightness of light sources is increasing, and the temperature of their fluorescent material tends to increase. Therefore, a ceramic sintered body with high heat resistance is receiving attention as a fluorescent material. For example, a fluorescent material composed of a ceramic sintered body prepared by adding cerium (Ce) to yttrium aluminum garnet (YAG) shows yellow fluorescence and is used in combination with a blue LED or a blue LD to obtain white light. Patent Document 1 describes a ceramic sintered body in which a fluorescent phase represented by YAG:Ce is contained in an alumina (Al2O3) matrix phase.